(1) Field of the Invention
The invention relates to the airline industry in general and more specifically to improved propulsion systems for powering aircraft airframes.
(2) Description of the Related Art
As illustrated in FIGS. 1-2, commercial aircraft airframes 20 have a central fuselage 22 for carrying passengers and cargo, wings 24 extending outwardly from the fuselage 22 for providing lift, a rear mounted tail 26 and variable surfaces 28 for control. Typically, the airframe 22 is powered by one or more propulsion systems 30 mounted in various arrangements. The propulsion systems 30 may be mounted alongside the rear portion of the fuselage 22 (FIG. 1), mounted in the tail 26 or they may be suspended beneath the wings 24 from pylons 32 (FIG. 2). For example, McDonnell Douglas DC-9 and MD-80 style aircraft models have propulsion systems 30 mounted alongside the rear portion of the fuselage 22. McDonnell Douglas MD-11 and Lockheed L-1011 style aircraft models have propulsion systems 30 mounted under the wings 24 and inside the tail 26. Boeing B-747 and Airbus A-380 style aircraft have propulsion systems 30 suspended beneath the wings 24 only. Although these aircraft are exemplary, other aircraft styles exist. The propulsion systems 30 are usually streamlined with outer cowlings 34, oftentimes referred to as nacelles, to reduce aerodynamic drag. In each of the above prior art propulsion systems 30, a bladed propulsion element such as a propeller or bladed propulsion element is driven by a dedicated gas generator core 38.
Propulsion system 30 fuel burn and weight are extremely important to all airline operators. Estimates indicate that aviation fuel charges represent approximately thirty percent of an operator's yearly recurring costs. Since the propulsion systems 30 run for extended periods while an aircraft is in flight, any reduction in fuel burn or weight can save an operator considerable money over the lifetime of the propulsion system 30.
As illustrated in FIG. 3, a conventional propulsion system 30 has a large diameter bladed propulsion element 36 that is driven by a gas generator core 38. These propulsion systems 30 are referred to as high bypass ratio turbofans and, due to their low fuel burn, are now commonplace in the commercial airline industry.
The bladed propulsion element 36 of the lower half of FIG. 3 is directly coupled to a turbine 40 in the rear of the propulsion system 30 via a primary shaft 42. Expanding core gases 44 drive the turbine 40, thus providing the necessary energy to drive the primary shaft 42, which drives the bladed propulsion element 36. Since the bladed propulsion element 36 is directly coupled to the turbine 40, the bladed propulsion element 36 rotates at a relatively high speed. The high speed of the bladed propulsion element 36 produces very high tensile loads on the other rotational components, so these components are made considerably larger and heavier to prevent failure. The added thrust of a larger bladed propulsion element 36 is often negated by the weight increase, making ever-larger bladed propulsion elements 36 an unattractive alternative for reduced fuel burn. The speed at which a large bladed propulsion element 36 rotates is also too fast for optimum aerodynamic efficiency and wing 24 to runway clearance is a limitation as well.
An alternative propulsion system 30 architecture is called a Geared TurboFan (GTF) and is shown in the upper half of FIG. 3. Here, the turbine 40 drives a large bladed propulsion element 36 at a slower speed through a reduction gearbox 46. The turbine 40 drives the bladed propulsion element 36 without excessively loading the rotational components. By turning the bladed propulsion element 36 at a slower speed than the turbine 40, the bladed propulsion element 36 operates at its optimum aerodynamic efficiency as well. Although the GTF provides benefits over other high bypass ratio turbofans, the wing 24 to runway clearance ultimately limits the maximum obtainable bladed propulsion element 36 diameter.
While the above-described propulsion systems 30 provide reduced fuel burn, there are still limitations to the size of the bladed propulsion element 36 that may be driven. The larger bladed propulsion element 36 size and reduction gearbox 46 also increases the weight of the engine. Improved propulsion systems 30, which further reduce fuel burn and weight over the current state of the art, are therefore needed.